The present invention relates to trimmer capacitors and, more particularly, to trimmer capacitors of a small size which may have adjustments in the capacitance value thereof changed reliably, and do not change in height when the capacitance value is changed.
Trimmer capactiors of compression type, such as that described in U.S. Pat. No. 3,943,417, typically include an earth electrode secured to an insulating frame, a dielectric member, and another electrode having an end portion attached to an insulating frame and a curved plate portion resiliently displaceable. The plate portion opposes the earth electrode through the dielectric member, and a pressing plate is superposed onto the plate portion. A screw shaft is inserted through the electrodes, the dielectric member and the pressing plate. The rotation of the screw shaft causes the pressing plate to be moved in an axial direction to displace the plate portion and thereby change the distance between the two electrodes. The capacitance value can thereby be varied.
In such conventional trimmer capacitors, there often has been a large amount of play between a threaded opening for the earth electrode and the shaft portion of the screw shaft in order to allow the screw shaft to be rotated smoothly. Therefore, when a screw driver or the like is first engaged with or dis-engaged from the screw shaft, if the axial pressing load by the screw driver changes, the distance between the earth electrode and the curved portion of the other electrode may change to vary the capacitance, even though the screw shaft has not been rotated. Thus, accurate adjustment of the capacitance value is extremely difficult. In addition, the capacitance value may drift a relatively large amount after being set, with obvious disadvantage. Therefore, in order to remove these disadvantages, the threaded opening may be formed in an insulating frame made of a plastic material and have a closed portion tapped by the screw shaft, to thereby lessen play. Such a type of trimmer capacitor is disclosed in U.S. Pat. No. 3,943,417. However, in this kind of trimmer capacitor, a deformation may form in the insulating frame due to the flexibility of the plastic material thereof when the rotating torque of the self-tapping screw shaft is large, such as near the maximum capacitance value of the device. In such case, the distance between the electrodes may be changed by the axial load in trying to rotate the screw shaft, and the capacitance value set may have a large drift, making it further difficult to adjust the capacitance value reliably. In order to prevent such deformation of the insulating frame, the capacitance value may be adjusted in the vicinity where the fastening torque is small. In such case, however, the maximum value of capacitance cannot be made large.
Also, many conventional types of trimmer capacitors move upwards and downwards in accordance with the rotation of the screw shaft, so that the length or height of the trimmer capacitor is disadvantageously changed. Accordingly, such capacitors are not suitable for use in the electric products requiring a thin shape.
Thus, it is an object of the present invention to provide a trimmer capacitor in which the distance between the electrodes may not be changed even by the pressing load of a screw driver and the like.
It is other object of the present invention to provide a trimmer capacitor whose height or length is not changed during adjustment of the capacitance value.
It is still other object of the present invention to provide a thin trimmer capacitor.
Therefore, in accordance with the present invention, the screw shaft is adapted to be constantly supported by an earth electrode surface, and only the pressing plate is designed to be moved upward and downward. Further objects and advantages of the present invention will be more apparent in conjunction with the drawing illustrating one embodiment of the present invention and the explanation thereof.